The development of a compact, tunable, hard x-ray source would have profound and wide ranging applications in a number of areas. These areas include x-ray diagnostics, medical imaging, microscopy, nuclear resonance absorption, solid-state physics and material science.
Currently, varieties of x-ray generators exist. The most modern devices are generally based on one of three methodologies: laser and discharge plasmas, electron impact sources, and synchrotron. The spectrum of these sources can be divided into two categories: characteristic x-rays and continuum x-rays. The characteristic x-ray sources are dependent on the particular atomic structure of the gas or target material in use. Among all the types of x-ray sources, only synchrotron produces continuum radiation.
The main interest in laser-generated plasma is directed towards inertial confined fusion. Recently, they have also gained interest as sources of (V)UV and x-rays. Laser-generated plasmas emit photons in an energy range, which can extend from visible light to hard x-rays. The observed emission spectrum is characteristic of a high-temperature, short-lived, high-density plasma. The sources produce a spectrum of x-rays centered about characteristic lines of the material.
In a laser-generated plasma x-ray source, when a high-power pulsed laser is focused on a (solid) target, a plasma is created. After the laser pulse terminates, the plasma cools extremely rapidly due to rapid thermal conduction, electron energy loss to ions, and expansion of the plasma into the surrounding vacuum. Cooling of the electrons at high density leads to fast recombination, quenching of the highly excited states, and a termination of the x-ray emission. The choice of target material controls the intrinsic range of the spectral output determined by the ionization states of the target material. Details of the spectral distribution are highly dependent on the target material (e.g., carbon, aluminum, titanium, copper, zinc, molybdenum, tin, tungsten, and lead) and other parameters (target thickness and source size).
Plasma discharge systems have been suggested as sources of high brightness x-ray radiation. Most of these devices (the gas puff J. Pearlman an J. C. Riordan, J. Vac. Sci. Technol. 19, 1190 (1981), plasma focus Y. Kato, et al, Appl. Phys. Lett. 48,686 (1986), and hypocycloidal pinch K. S. Han, et al, Bull. Am. Phys. 31, (1986)) are variations of the Z-pinch geometry. In Z-pinch devices, a high current is produced on the outer edge of a cylindrical volume of gas using a pulsed electrical driver such as a fast capacitor bank. The resulting JxB force accelerates the plasma shell radially inward to form a very high-temperature plasma on-axis which emits characteristic thermal radiation in the soft x-ray region.
The conventional electron impact sources use a suitable target material that is bombarded by a high-energy electron beam. These sources produce a broad spectrum of x-rays centered about characteristic lines of the material.
Synchrotron radiation is the electromagnetic radiation emitted by electrons moving at relativistic velocities along a curved trajectory with a large radius of curvature, for example, several meters to tens of meters. The energy of the photons ranges from a few electron volts to 10.sup.5 Ev. This corresponds to the binding energy of electrons in atoms, molecules, solids, and biological systems. Thus, synchrotron radiation photons have the right energy to probe the properties of such electrons and of the corresponding chemical bonds to understand their physical and chemical properties. The uses of electron accelerators as sources of synchrotron radiation have grown enormously during the last two decades. Unique features such as tunability and wide x-ray spectrum tend to render the synchrotron irreplaceable for many applications.
Presently, third generation synchrotron sources are being pursued that are based on high-energy electron storage rings and bending magnets. A typical electron accelerator can be tuned to emit synchrotron radiation in a very broad range of photon energies, from microwaves to hard-x-rays. Thus, it provides electromagnetic radiation in spectral regions for which no other usable sources exist, e.g., most of the ultraviolet/soft-x-ray range. Furthermore, it is by far the best source of hard-x-rays, even though other sources exist for this range. The system has met most application needs, but fails with respect to physical size and cost. They are inevitably large and expensive devices requiring complex supporting facilities. The current machines are very large and costly with tens to hundreds of millions of dollars. The nature of synchrotron x-ray sources means that they are expensive, remote multi-user facilities, and are therefore not suited for use with a laboratory scale. The alternative x-ray sources, such as electron impact systems, laser and discharge plasmas, cannot match synchrotron in terms of its tunability and continuum x-rays.
An object of the invention disclosed is to provide a small compact tunable x-ray source.
Another object is to provide a compact tunable x-ray source for laboratory use. For applications where a relatively small sample is practical, the availability of a laboratory-scale source would be very advantageous.
Another object is to provide a compact tunable x-ray source for security inspection applications such as more sensitive balcale x-ray inspection systems.